The oligomerization of lower olefins, particularly C2 to C6 olefins, to produce higher molecular weight olefinic products is an important commercial process useful in, for example, the production of fuels and precursors for plasticizers, surfactants, and freeze point depressants for lubricating oils.
For example, approximately 70 units exist world wide for the purpose of oligomerizing olefins (typically mixtures of propylene and butenes) e.g., from Fluid Catalytic Cracker (FCC) unsaturated gas plants and/or steam crackers to gasoline and or distillate. These plants employ multiple reactors filled with solid phosphoric acid catalyst (sPa). SPa catalyst typically produces 500 to 1500 weight units of oligomer per weight unit of catalyst and then reaches the end of its useful life. As a result, most operators are required to shut down and reload catalyst into a reactor every 3 to 10 weeks. The reactor is taken off line, refilled with fresh catalyst, and brought back on line. Reactor turnaround for sPa catalyst is particularly difficult. During the course of use, sPa catalyst agglomerates to form a single, solid block which must be water jetted or drilled out of the reactor. Although sPa catalyst is inexpensive (currently about $2/lb), catalyst cost to produce oligomer is high compared to processes with more productive catalysts such as hydrotreating catalysts, hydrocracking catalysts, FCC catalysts, ethylbenzene and cumene catalysts, xylene isomerization catalysts, etc. due to the large quantities of sPa catalyst required and the expense associated with shutting down and restarting reactors.
For many units, sPa catalyst useful lifetime is limited by the increasing pressure drop caused by the steady catalyst agglomerization and not by loss of too much catalyst activity. Because of these problems, operators of sPa olefin oligomerization units are careful to maintain operating conditions that maximize catalyst cycle length. The rate of sPa fouling is known to increase with increasing feed olefin concentration. Many sPa operators therefore dilute the olefin feedstock with a paraffin recycle to increase catalyst lifetime. Paraffin dilution decreases the capacity of the unit by taking up space in pumps, reactors, heat exchangers and distillation towers.
One example of a process that utilizes a solid phosphoric acid oligomerization catalyst is U.S. Pat. No. 6,025,533, which describes a process for the production of heavy oligomers by a combination of dehydrogenation and oligomerization.
It is also known that zeolites can be attractive replacements for sPa catalysts because of their unique selectivities in olefin oligomerization. In addition, zeolite catalysts in light olefin oligomerization service do not swell and fuse, and the pressure drop across the unit remains small and constant throughout the full catalyst cycle. Zeolite catalyst fouling is also typically independent of feed olefin concentration.
For example, U.S. Pat. Nos. 3,960,978 and 4,021,502 disclose the conversion of gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, into an olefinic gasoline blending stock by contacting the olefins with a ZSM-5 type zeolite. In addition, EP-B-746,538 discloses oligomerization of propene and butene to produce enhanced yields of the trimer using zeolites of the structure types MFI, TON, and MFS, such as ZSM-5, ZSM-22 and ZSM-57.
International Patent Publication No. WO 94/12452, published Jun. 9, 1994, discloses a process for producing a branched C4-C5 olefin by contacting a mixture of ethylene and a C3-C10 olefin with a molecular sieve selected from ZSM-22, ZSM-23, ZSM-35, ZSM-50 and SAPO-11 at a temperature of 200-700° C.
U.S. Pat. No. 4,919,896 describes the use of series reactors for oligomerization of olefins; a number of different zeolites, including ZSM-22, are proposed as catalysts.
U.S. Pat. No. 5,672,800 describes a process for oligomerization of C2-C12 alkene-containing feedstock having a water content of from 0.05 to 0.25 molar % over a zeolite catalyst.
U.S. Pat. No. 6,143,942 and International Patent Publication No. WO 95/22516, published Aug. 24, 1995, disclose an olefin oligomerization process comprising contacting a feed comprising at least one olefin under oligomerization conditions with a catalyst comprising at least one zeolite having a constraint index greater than 10, such as ZSM-22, and at least one zeolite having a constraint index of 2 to 10, such as ZSM-5 or ZSM-57, said zeolites being present in a proportion within the range of 10:90 to 90:10 by weight. Advantageously the two molecular sieves are in admixture but they can also be arranged in separate beds so that the feed passes through them in series. The feed can contain an inert diluent, such as a saturated hydrocarbon, in addition to said at least one olefin. For a feed comprising propene, a suitable diluent is said to be propane, advantageously in proportions of propene propane from 10:90 to 60:40, especially about 50:50 by weight.
However, not only are zeolite catalysts often expensive, but they tend to produce a different product slate than sPa catalysts when used to oligomerize olefins. For example, zeolite catalysts tend to produce more less branched products and more heavy molecules, with the product being more heavily weighted towards materials that are an integer multiple of the olefin monomer. Moreover, zeolite product carbon distribution is a strong function of conversion between 90 and 99% light olefin conversion. As conversion increases, distillate selectivities of greater than 30% can be obtained. SPa catalyst product number distribution remains nearly constant across this conversion range with less than about 10% selectivity to distillate.
In addition, zeolite catalysts can pose problems when used to oligomerize olefins under commercial, non-isothermal conditions. Thus high-olefin content (>65%) feedstocks containing propylene are among the most important feedstocks in the industry but oligomerization of these feedstocks is highly exothermic. When zeolite catalysts are used to process such feedstocks, large and unstable exotherms can develop anywhere in the reactor bed requiring reactor shutdown.
There is therefore a need for an oligomerization process in which catalyst lifetime can be improved and in which high olefin content feedstocks can be processed without diluents and without the production of uncontrollable exotherms.